It could be a fault with the water separator.  Do you have a water separating filter upstream of the engine-mounted filter?  If so, have you checked that for water?  If no water is present in the upstream filter also, check the sensor by checking for continuity with a multimeter across the wires from the sensor (remember to unplug the sensor first).  If you hold the water separator assembly upright and then turn it upside down you should get a change of state (i.e. continuity in one position, but not in the other) as the float moves.  I think you should get no continuity with the cup facing up (i.e. with the float in the down position) and continuity with the cup facing down (i.e. with the float in the up position).  If you don't get a change of state then the problem is either with the switch or with the float in the water separator assembly.  Did you tilt the engine up and down between starts when you experienced the alarm?  If not, the fault is likely to be with the switch as the float should not have moved.  You can order the parts yourself and replace them or get a professional to do it.

Let us know how you go.  If you need more help with parts please advise the exact model number of your engine (including the 2 or 3 letters after the horsepower rating) and the year of manufacture.

Old Tub

 Thanks Old Tub, really appreciate your detailed response. Planning to take the water separator apart from inside the motor as you mentioned. According to the service manual it doesn't seem too hard, will clean out and inspect, from the outside it doesnt seem to have water in it so I am thinking it may be a fault. Do i need to do anything special to re-prime orjust pump the bulb? Motor is a F150hp 4 stroke 09 model. Probably won't get a chance until Friday but will let you know how I go. Are you a boat mech? May need to pay you a visit if I have no joy, you offered more assistance than the service centre who I called today and payed $680 to Service my boat not so long ago. Thanks again, Matt

As your engine is a 2009 model is it still under warranty?  If so, once you have determined that it is definitely a fault with the water separator switch, you should take it into a Yamaha service centre and get them to fix it under warranty.  I am not a professional mechanic, just a keen amateur with a good mechanical and electrical knowledge who has spent a lot of time working on outboards, most of which weren't mine!  If I don't consider myself suitably experienced to comment on a problem then I won't, however  I have previously come across the same problem on an F150 that you have so I should be able to work you through it and it should be relatively simple.   Most dealers or service mechanics don't like doing phone or internet diagnoses because it's not good for business and they will have spent too much time fixing amateur disasters.

Unfortunately the manual won't tell you what a pain the filter bowl can be to get off.  Hopefully it won't be too bad for you as it will have been recently reinstalled during the service when the filter element will have been replaced.  Yamaha make a special filter wrench tool for removing the bowl, but a normal filter wrench should get it off, or you might be lucky and even manage to get it off with your hands.  Failing that, you can remove the whole assembly including the head, drain all of the fuel out and put it in the freezer for an hour or two, which sometimes makes the bowl easier to remove.  It is best to refill the filter assembly with clean fuel before reinstalling to eliminate as much air from the system as possible and to reduce the strain on your fuel pump.  Put some oil on the seal and on the filter assembly threads before replacement to make it easier to get off next time.  Reinstall the filter assembly onto the head, taking care not to cross the threads and that the seal is in place, and tighten it by hand.  When you are testing the switch for continuity as I described in my last post, remove the complete assembly including the head as one piece and test it as I described without taking it apart.

I am fairly sure the fuel water separator assemblies are the same on all of the post 2006 F150's, but if you end up needing parts advice can you please advise the exact model name of your engine.  If you look on engine clamp bracket on the port side their should be a primary identification label.  The engine model name will be identified as something like F150TLR. The location of the label will be shown at the start of the owners manual.

Old Tub

 Hi old tub thought I would post an update, I had a look as you mentioned at both the bulb in the motor and the canister, no water so then took to the Yamaha dealer, no water. Turned out to be an issue with the sensor- fixed under warranty so happy days. Thanks again for all your help. Good luck on the water

Good outcome and a perfect example of how a forum should work.